B-52 AT100 power supply(fixed) now chasing 60 hz

[j_bruce]

A guy in town heard that I had fixed a few amps for other locals and brought be a non working B-52 AT100.

Visual inspection showed a very clean, nearly untouched amp inside with R137 having burned free of the PC board and dropped out. I found this schematic of the power supply:

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/50051-

Obviously I need to replace the resistor, and I have one ordered, but is there a reason in the power supply that this might have happened? Is it possible/likely that a failed resistor is the only problem?

The B+3 feeds the reverb tube and the B+4 feeds the pre-amp tubes of both pre-amp channels. What else might I be on the look out for as I bring this back up to speed sans tubes on a limiter? I also notice that R140 between B+3 and B+4 is a 2 watt which makes me wonder if the 1 watt spec is a bit undersized. I ordered a 2 watt replacement for R137.

Thanks
Jerry

[Latole]

It is indeed possible that this 1 watt should have been a 2 watt. You don't specify if the resistor measures the right ohms. The resistor could have burnt through without causing a problem.

These resistors may be soldered too close to the circuit board preventing the heat from escaping.
A picture would help.

What is the problem with the amp ?

[shooter]

that's not an amp for the faint at heart! 
I've had one cracked open a few times.  be real careful with the bias n bias circuit.


here's a couple versions.  I took the PDF to Staples, they blew up the schematic to eye friendly size.   

[Latole]

https://wgsusa.com/node/2724

https://www.unbrokenstring.com/beer-head-b52-at100-head/

[sluckey]

Obviously I need to replace the resistor, and I have one ordered, but is there a reason in the power supply that this might have happened? Is it possible/likely that a failed resistor is the only problem?

Resistors usually burn up due to excessive current. It's rarely the fault of the resistor. Measure resistance between Node B+3 and ground looking for low or zero ohms. C88 and C89 are suspects. Also look at the reverb driver V10-1 circuit. Does R59 show signs of overheating? Node B+3 also feeds V11 (send/return loop) but unlikely to burn up R137.

[j_bruce]

Here I will try to attach a photo:

[Latole]

What is the problem with the amp ?

Picture of the resistor ?
How many you read ? Or resistor is broken in half ?

[j_bruce]

You can see in the photo that the resistor burned its way right out of the board, it was not inside when the amp got to me.

Owner said it makes no sound but does pop when he turns the A/AB/SS selector switch.

 Shooter I do not plan to mess with anything that I don't have to!

sluckey thanks for the test points, I was suspecting C88 and will check all of the points you mentioned.

Just went to check R59 and it is burnt too! sluckey for the win. So that is the next thing to deal with. I better check that tube as well. Owner said after it quit he bought new tubes to replace the tubes that did not "light up" at that point. All the 12ax7s are groove tubes and look pretty new, I have a few extra 12 ax7 as well.

Waiting for resistors and then will report back.

Cheers!
Jerry

[Latole]

Thank for the answer.

Repairs are not easy after owner mess it ........

[sluckey]

What is the problem with the amp ?

Picture of the resistor ?
How many you read ? Or resistor is broken in half ?

Look at the pic.

[Latole]

What is the problem with the amp ?

Picture of the resistor ?
How many you read ? Or resistor is broken in half ?

Look at the pic.

Thanks but he had already answered me.

[shooter]

Owner said it makes no sound but does pop when he turns the A/AB/SS selector switch.

that switch is bad on the last one I looked at.  Only SS position intermittently worked.  the owner just wanted it "to work"  So I bypassed the switch, hardwired it to SS "position".  bias is also on that switch.

[j_bruce]

OK,

Parts arrived and I swapped out the bad resistors. Tested for issues everywhere mentioned above and nothing seemed off. Started up sans tubes all happy B+ voltages all made sense. Put the tubes back in on a limiter and everything worked as expected.

But, a solid 60 hz hum. Owner stopped by to check it out and liked the overall sound of the amp but wanted to get rid of the hum. So I need to order some new caps for the power section, he said change them all since the hard part is getting the power supply board out.

Are there any other potential sources that I should look at once I open it back up?

My hunch is that C88 or C89 is bad enough to create the noise and some set of conditions made it eat the resistors at some time. It's a pretty nice sounding amp, the hi gain side is very load and does what it should. The clean side has a decent tone as well so once he learns the settings it should do well.

Thanks
Jerry

[tubeswell]

If R137 fried off the board, there may be other damage, either to PCB traces, or heat-sensitive parts nearby (electrolytic caps overheating). Or, when whatever-it-was-that-failed (tubes?) failed the first time, there may've been some kind of transient voltage spike through the PT, which could have upset some secondary PS circuit. Are there any relays in the amp? (And are the PS filter caps for those all working properly?) Can't tell without the whole schematic. (I see some spade-terminal connectors in the pics. (The white wires - are those for heaters?). The surfaces of these contacts can oxidise over time, creating resistance across the connection, and if there is a current surge - like from heaters at startup - resistance like that can (depending on the current) cause a voltage change that can be inducted back through the PT into other windings/PSs. Not saying this has happened in this case - just guesswork)

[j_bruce]

The board did not sustain any heat damage. The resistor and related caps are all on large pads of copper on the board so I think that helped a lot.

Lots of relays in this thing, it's a complex unit with class A, class A/B and solid state modes selected by a switch on the back.

If there are problems there I may have to pass on chasing beyond the PS caps. There is one cap that was right next to the bad resistor so I will check that one out.

Thanks,
Jerry

[shooter]

Are there any relays in the amp?

it's a treasure trove of relays and IC's!


did you take the PRE signal out at the "send" jack and determine if the 60HZ is also present???

[j_bruce]

Are there any relays in the amp?

it's a treasure trove of relays and IC's!


did you take the PRE signal out at the "send" jack and determine if the 60HZ is also present???

I will check that Shooter thanks!

[j_bruce]

So, I got back on this today. replaced the PS caps, maybe not the problem but something caused the burned resistor..

After that the sound was improved but still 60 hz.

So I started going down the signal path with my scope to have a look for good signal then bad signal. noise seems to be coming in between V1-2 and V2-2. Interestingly enough when I scope before C14 95% of the 60 hz goes away, and when I scope after R21 but before V2-2 the noise goes silent and the amp sounds great.

So now I have two nice clues, but do not know where that takes me.

Any help is appreciated,
Jerry

I found the schematic here:

https://www.eserviceinfo.com/preview.php?fileid=67976&file=AT_100_Schematic_Diagram.gif

[shooter]

does the noise change when switching between clean and Overdrive?

[j_bruce]

does the noise change when switching between clean and Overdrive?

some, but not entirely. The overdrive is noisier but the basic 60 hz is there on both.

Jerry

[j_bruce]

I have been working my way through this now and then and have gotten rid of all of the power supply noise and found a chinese tube that was introducing some 60 hz as well.

So at this point the clean channel is perfect. Super quiet even with everything dimed out. So That tells me the power supply is settled and V1 is not the culprit (I have swapped V1 just to be sure).

The gain channel still has a 60 hz noise but not really noticeable until you turn the gain up past half way or so. Then I discovered that if I turn all of the tone knobs to zero the noise is gone and so is the signal. The bass seems to have a more significant effect on this: if the bass is on a bit there will be signal; if the bass is off then it all goes quiet. This seems to be a big red flag!!

could this be as simple as contact cleaner for the pots? It will take me a bit to pull the board so I can clean the pots, willing to do that if it makes sense but rather not if I should be looking further. I tracked down all of the relevant components on the boards and none show any visual issue, though I know that is not the whole story. I will freely admit that tone stacks are still fuzzy for me. I totally get high pass and low pass filters but the interactions leave me searching for some help.

Thanks again for any help, I appreciate the learning opportunities that you all have provided for me over the past couple of projects.

Jerry

[sluckey]

Then I discovered that if I turn all of the tone knobs to zero the noise is gone and so is the signal.

That's exactly how that TMB tonestack should work. Same with any Fender TMB.

[j_bruce]

Then I discovered that if I turn all of the tone knobs to zero the noise is gone and so is the signal.

That's exactly how that TMB tonestack should work. Same with any Fender TMB.

Hmm. Interesting. I have never played an amp that had this type of tone stack so I thought this was odd. I play a Kalamazoo Model 2 for harp.

I guess I will keep digging through and see if I can find the source. It is clearly only on the hi gain side. I think the amp is perfectly playable as it is but that rock steady 60 hz is bothering me. I don't expect hi gain to be dead quiet, but.

Thanks
Jerry

[shooter]

if I turn all of the tone knobs to zero the noise is gone

so;
the noise is left of the TS, mostly
60Hz tells me someone's coupling filaments into the signal lane without permission.

[tubeswell]

if I turn all of the tone knobs to zero the noise is gone

so;
the noise is left of the TS, mostly
60Hz tells me someone's coupling filaments into the signal lane without permission.

Rectifier diode switching transients being coupled through the PT? (Relay PS rectifiers?) - guesswork

[j_bruce]

if I turn all of the tone knobs to zero the noise is gone

so;
the noise is left of the TS, mostly
60Hz tells me someone's coupling filaments into the signal lane without permission.

I agree, but who is the guilty party?

and is it really a party if someone is guilty??

 

[j_bruce]

if I turn all of the tone knobs to zero the noise is gone

so;
the noise is left of the TS, mostly
60Hz tells me someone's coupling filaments into the signal lane without permission.

Rectifier diode switching transients being coupled through the PT? (Relay PS rectifiers?) - guesswork

Tubeswell: This and Shooter's comment got me looking at the schematic and thinking "who could be to blame that effects the hi gain side but not the clean?"  There is a number of diodes and caps in that part of the circuit that would be isolated out of the signal chain when switched to the clean channel so I at least have some targets to attack. I am going to do some scope testing and see if I can find the noise, I have a strategy in mind.

is there an easy way to test diodes on the pc board?

Thanks
Jerry

[sluckey]

is there an easy way to test diodes on the pc board?

That depends on the circuit around the diode. You cannot check a diode that is connected across a relay coil because of the low resistance of the coil. You can check a diode that is connected across a 1M resistor. You just have to examine the circuit to determine if the diode check will be valid, then use your diode check function of your meter to test. Or if you have an analog meter, check front to back resistance ratio. If the diode checks OK in circuit, move on. If it checks bad or you don't understand the reading, then unsolder one end of the diode to make a valid check.

[shooter]

I am going to do some scope testing and see if I can find the noise

doublecheck it's 60hz, not 120hz


the relays are rectified DC so 120hz most likely.


I believe you can work backward - grounding grids to see where the noise goes away
V3-2 Pin 7
v3-1 Pin 2
V1-2 Pin 7
WAIT for a 2nd opinion on that, it's real early n my coffee isn't ready yet!

[j_bruce]

I am going to do some scope testing and see if I can find the noise

doublecheck it's 60hz, not 120hz

Shooter I have a guitar tuner app on my phone, it says 60 hz, -50 cents

so I am believing 60 hz, I can scope check that though.

[j_bruce]

Finally got this thing done,

touched up a couple solder joints around V2 and V3 and she is down to a bit of background noise. Owner is happy.

Thanks for the help, always a pleasure learning by doing with expert guidance when needed!

Jerry

[shooter]

 

is it really a party if someone is guilty??

Owner is happy

Asked, and answered
way to stick with it